Radio telecommunications system with improved use of timeslots

ABSTRACT

A packet control unit for a radio telecommunications system such as GPRS has a radio link control/medium access control blocks scheduler which allocates a number of temporary block flow queues to a timeslot for transmission to a mobile system; a fixed allocation table is provided having allocation slots corresponding to RAC blocks, and the RAC blocks scheduler reads the table cyclically and allocates slots according to the reading. Timeslots are requested later during increased demand conditions and are released earlier during decreasing demand conditions than in currently known arrangements.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority of European Patent ApplicationNo. 00301868.6, which was filed on Mar. 7, 2000.

FIELD OF THE INVENTION

[0002] This invention relates to a packet-based radio telecommunicationssystem, such as GSM (Global System for Mobile Communications), havingimproved traffic control, especially over the air interface, and relatesespecially to a Packet Control Unit (PCU) in the Base TransceiverStation (BTS) having improved interworking with Mobile Stations (MSs)and with the Serving GPRS Support Node (SGSN).

DESCRIPTION OF THE RELATED ART

[0003] In a known packet-based system, a practical problem is that thePCU can only react on demand to packets received, and is not aware oftraffic it will receive in future. The question is how to allocateresources on the air interface which carries GSM voice calls, controllinks, and timeslots. In current systems, the bit rate, the priority andthe delay applied to each GPRS connection can be varied, but thisrequires considerable computing and messaging resources between the MSand the BTS, between the BTS and the PCU, and between the PCU and theSGSN.

[0004] The blocking rate of a packet-based system is the proportion ofusers who are trying to access the system but cannot immediately beserved; the rate can never be zero, which implies infinite resources,and practical experience shows that a blocking rate of 5% generates asubstantial level of caller complaints. A blocking rate of 2% has beenfound to be acceptable, and a maximum number of GPRS transmissions canthen be served. It is desirable to improve the traffic control of thesystem by using the timeslots more efficiently while maintaining theblocking rate of the system at an acceptable level.

SUMMARY OF THE INVENTION

[0005] According to the invention, a packet control unit for a radiotelecommunications system comprises a down link requests scheduler; abase station system virtual connection flow controller; a mobile systemflow controller; a first data transmission means to exchange data with aplurality of mobile systems; and a second data transmission means toexchange data with the serving support node of the radiotelecommunications system; there being associated with the firsttransmission means a queuing means to provide a plurality of temporaryblock flow queues, and a radio link control/medium access control blocksscheduler having allocation means arranged to allocate a temporary blockflow queue to a timeslot for transmission to a mobile system only whenthat queue contains data. Optionally each timeslot is essentially filledwith data before transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The features and advantages of the present invention will becomemore apparent from the detailed description set forth below when takenin conjunction with the drawings wherein:

[0007]FIG. 1 is a schematic illustration of the BSS part of the GPRSradio telecommunications system;

[0008]FIG. 2 is a schematic illustration of the packet control unit of aradio telecommunications system;

[0009]FIG. 3 indicates two cycles of data transfer for the TBF queues;

[0010]FIG. 4a illustrates the resulting data stream on the link to theMSs in block form;

[0011]FIG. 4b illustrates the scheduling table allocation; and

[0012]FIG. 4c illustrates the resulting interleaved data stream to theMSs.

DETAILED DESCRIPTION

[0013] In FIG. 1 a General Packet Radio Service (GPRS) system 10comprises a GPRS Backbone System (GBS) 12 containing a Serving GPRS Node(SGSN) 14 which is connected by an interface link Gb 16 to a PacketControl Unit (PCU) 18 within a Base Station System (BSS) 20. The PCU 18is connected to a number of Mobile Systems (MS) 22 through an interfacelink Um 24.

[0014] In FIG. 2 the PCU 18, the link Gb 16, and the link Um 24 areshown. Between the links 16 and 24 there is a Down Link (DL) requestsscheduler 26, a MS flow control unit 28, a Base Station System VirtualConnection (BVC) queue scheduler 30; a BVC flow controller 32, and aRadio Link Control/Medium Access Control (RLC/MAC) blocks scheduler 34.

[0015] Although the uplink is not illustrated as it is not activelyinvolved in application of the present invention, it will naturallyexist.

[0016] In normal operation at normal traffic flow rates, a new call to aMS 22 is received over the Gb link 16, and the first Protocol Data UnitDL-UNITDATA PDU passes to the queue 38 of the DL requests scheduler 26.The scheduler 26 instructs the RLC/MAC blocks scheduler 34 to allocatecapacity, and a Temporary Block Flow (TBF) queue, such as queue 42, isset up for the called mobile. Signals pass over the Um link 24 to thecalled mobile, which returns acknowledgement signals, ACK, over link 24to the queue 42. The scheduler 34 fetches the next DL-UNITDATA PDUwhich, because the MS is now known, passes through the BVC queuescheduler 30 into the BVC queue 36, and then to the appropriate TBFqueue, queue 42 in this example.

[0017] The scheduler 26 assigns logical resources to one of the TBFqueues 40, 42, and instructs the RLC/MAC block scheduler, message(2)-(2). The processor 28 reads from the second DL-UNITDATA PDU themulti slot capacity of the mobile.

[0018] One TBF queue is allocated to each active MS. Every MS with anactive TBF queue has two logical queues; one is for Logical Link Layer(LLC) data PDUs, and one is for LLC signaling PDUs. The length of theBVC queue 36 is the sum of all the MS queues including LLC data andsignalling.

[0019] The BVC queue 36 is provided towards its output end with twotimeslot triggers TS(l) and TS(u) which function in the conventionalway, that is, if the queue 36 exceeds the upper timeslot trigger TS(u),the DL requests scheduler 36 sends a request via interface Gb 16 to theSGSN 14 for an additional timeslot to be allocated. If an additionaltimeslot is allocated, the queue shortens as traffic throughput isincreased, and the queue length falls below the upper trigger TS(u). Thesignal from the queue 36 to the scheduler 26 is indicated as message(3)-(3).

[0020] In the arrangement of FIG. 2, the major components illustratedperform their conventional functions as follows:

[0021] The DL requests scheduler 26 allocates resources, i.e. it decideswhich is the next MS 22 to be connected over the link 24; it requestsadditional timeslots when the timeslot trigger TS(u) is activated onconnection (3)-(3),and it returns timeslots when they are no longerrequired.

[0022] The RLC/MAC blocks scheduler 34 allocates capacity, i.e. itallocates timeslots in response to instructions from the DL scheduler36, message (2)-(2); it fetches the next DL PDU when the TBF queue 40,42 (or that MS is below a predetermined threshold (precedence is givento LLC signaling PDUs). The scheduler 34 also divides the LLC PDU intoblocks, sets up transmission windows and retransmissions of blocks foreach TBF queue 40, 42, and drops the TBF after a predetermined number ofresent transmissions; on termination of a call it also signals the endof a TBF queue to the DL scheduler 36, message (1)-(1).

[0023] The DL requests scheduler 26 and the RLC/MAC blocks scheduler 34are related as follows; the scheduler 26 assigns logical resources toeach TBF queue 40, 42 on a per timeslot basis, i.e. an allocation tableis set up, while the scheduler 34 dynamically maps these logicalresources to physical resources. Looked at another way, the scheduler 26decides which mobile 22 will next be connected, and the scheduler 34decides which block of information is sent to which mobile.

[0024] The BVC queue scheduler 30 selects the appropriate queue for amobile system. It directs each DL-UNITDATA PDU to that queue. Each MSqueue is divided into an MS data queue and an MS signalingqueue—conveniently via use of two pointers in a common buffer.

[0025] Data flow over the Gb link 16 is controlled by use of the leakybucket algorithm run within the SGSN 14 (FIG. 1). Values of Bmax, themaximum bucket capacity and R the leak rate, are calculated by both theBVC flow control processor 32 and the MS flow control processor 28 indifferent circumstances, and the values sent over the Gb link 16.

[0026] The following assumptions are made about the SGSN 14;

[0027] 1. The MS flow control values sent to the SGSN by the MS flowcontrol processor 28 are valid until the SGSN 14 receives a new valid MSflow control message or until the hysteresis timer Th expires. The PCU18 knows the timer value which the SGSN is using and is arranged to senda new MS flow control message every Th seconds less a short tolerancetime.

[0028] 2. If the SGSN 14 has not received an MS flow control message fora particular MS, the SGSN uses the default flow control values as sentin every BVC flow control message. The SGSN never uses internallygenerated MS flow control values, when these initial values are sent inthe BVC flow control message.

[0029] 3. The SGSN 14 sends the MS Radio Access Capability (a classmark) in each DL-UNITDATA PDU.

[0030] 4. The SGSN 14 keeps (logical) separated queues for both MS flowcontrol and BVC flow control.

[0031] Turning now to detailed consideration of the inventive feature,the RLC/MAC scheduler 34 controls the TBF queues 40, 42 etc by settingup an allocation table having 128 scheduling slots 0-127. Each slotcorresponds potentially to one RLC/MAC block. This is shown in FIG. 3 inwhich three temporary block flows TBF 1, 2 and 3 are considered, eachset of allocated slots being a different length and being shown shaded.As is highly probable in practice, many of the slots are unallocated.Since the DL requests scheduler has the same number of slots and thesame type of scheduling table, no translation is needed in communicationwith the RLC/MAC scheduler 34.

[0032] A pointer P reads each slot and the scheduler sends data for theappropriate TBF. The arrangement according to the invention is such thatif there is no data to send for a particular TBF, the scheduler skipsthose slots and looks for the next TBF having data to send. After slot127, the pointer returns to 0, i.e., scheduling is circular.

[0033] In FIG. 3, in Cycle 1, the pointer P starts at slot 0 and findsthat all of the slots 0-10 of TBFI have data, this data is then sent bythe RLC/MAC 34. At slot 11, there is no data, and the scheduler movesthe pointer to the next data-containing slot, i.e., slot 16. TBF2 hasdata to send, and slots 16 to 19 are read; TBF3 has data to send inslots 20 to 27, and these data are sent. At slot 28 there is no data,and the scheduler scans all the further slots until the pointer reaches0, when Cycle 2 of reading the scheduling slots begins.

[0034] In Cycle 2, TBF1 now has no data to send, but TBF2 and TBF3 havedata in slots 16 to 19 and 20 to 27 as before, which is read and sent.

[0035]FIG. 4a shows the combined data; no RLC/MAC blocks are wasted,except that a small amount of radio source is wasted due to the internalfragmentation of the last, partial block of TBF1 in slot 10. The numbersof the scheduling slots are maintained to improve readability.

[0036] It will be clear that consecutive cycles can have differentlengths and will normally be shorter than 128 scheduling blocks. Theallocation of TBFs is shown as consecutive for simplicity.

[0037]FIG. 4b shows a real allocation in a scheduling table; theallocation is as follows:

[0038] TBFI=1 (16 slots)

[0039] TBF2=2 (4 slots)

[0040] TBF3=3 (8 slots)

[0041] Spare=grey (100 slots)

[0042] It can be seen that the scheduling slots for each TBF are notconsecutive.

[0043] The allocation table is updated only when a user starts or endstransmission.

[0044]FIG. 4c illustrates the resulting data stream on the Um link 24;there are no spare RLC/MAC blocks, thus the use of simple statisticalscheduling according to the invention results in highly efficient use ofthe Um link 24 and full use of every timeslot. Thus a timeslot can bereleased more quickly than would be the case in currently knownarrangements, and a timeslot will be requested at a later stage inincreasing demand conditions than would be the case in currently knownarrangements.

[0045] A general result of application of the invention is that there isfair scheduling of resources among the MSs, and different bit raterequirements for coexisting TBFs are served; this is achieved withoutany real time control of throughput, and only by reading of a stableallocation table.

[0046] While this invention has been particularly shown and describedwith reference to a preferred embodiment thereof, it will be understoodby those skilled in the art that various changes and modifications inform and details may be made without departing from the scope and spiritof the invention as defined in the appended claims. Accordingly, it isintended that the appended claims cover any such modifications orembodiments that fall within the scope of the invention.

1. A packet control unit for a radio telecommunications systemcomprising a radio link control/medium access control blocks scheduler;first data transmission means to exchange data with a plurality ofmobile systems; and second data transmission means to exchange data witha serving support node of the system; there being associated with thefirst transmission means a queuing means to provide a plurality oftemporary block flow queues and a fixed allocation table havingallocation slots corresponding to radio access control blocks, the tablebeing cyclically read by the radio access control blocks scheduler, andradio link control/medium access control blocks being reserved inaccordance with the cyclical reading.
 2. The packet control unit ofclaim 1 in which the allocation table is prepared by a downlink requestsscheduler in the unit.
 3. The packet control unit of claim 2 in whichthe allocation is arranged so that every radio block on a timeslot isessentially filled with data before transmission.
 4. The packet controlunit of claim 3 in which each temporary block flow queue is allocatedthe full capacity of a timeslot when the queue is the only queue activein that timeslot.
 5. The packet control unit of claim 4 wherein theradio access control block scheduler 34 is arranged to achieve thedesired bit rate without real-time control.
 6. The packet control unitof claim 4 wherein the pointer (P) is arranged to jump from the lastitem of data in one temporary block flow queue (TBF1) to the nexttemporary block flow queue (TBF2) in the cycle which contains data.